LDH[unreadable]k[unreadable] is a novel isozyme of lactate dehydrogenase which we have identified in anaerobically shocked rat fibroblasts and in nonrat cells transformed by the Kirsten and Harvey murine sarcoma viruses. It is composed of 56 kilodalton subunits or 35 kilodalton and 22 kilodalton fragments. It is readily isolated from and distinguished from other isozymes in LDH, although we have found it as a contaminant of commercial LDH-5. In biochemical studies of LDH[unreadable]k[unreadable], we find that it has a high affinity for specific nucleic acids. We find that the 5',5'-dipurine nucleoside tetraphosphates Ap4A and Gp4G are exceptionally potent noncompetitive inhibitors of LDH[unreadable]k[unreadable] activity. LDH[unreadable]k[unreadable] acts in a coupled reaction with FMN. Human tumors contain high levels of LDH[unreadable]k[unreadable] activity ranging from 10-\to 500-fold higher than the levels in adjoining nontumor tissue. LDH[unreadable]k[unreadable] has been detected in the serum of over half of all patients examined (greater than 500) but is present in very few noncancer patients. Serum LDH[unreadable]k[unreadable] expression correlates with the presence of metastatic cancer. Studies of LDH[unreadable]k[unreadable] expression in normal tissues have revealed that the retina also expresses high levels of this enzyme. This is seen with mammalian retina but not with retinas of lower species. (A)